Care to explain the mechanics behind the greeks and a hypothetical arb scenario where this would be the case? Would be much appreciated

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Financing arbs like conversions, boxes and rolls (especially the latter). Theta is unimodal, but gamma(dgamma) can flip modality if the edge(spread) on rates, relative to un-leveraged public rates (LIBOR), is large enough. Gamma will always be fractional in exposure when compared to rho.

It's not a practical concern unless you're in danger of pinning on certain arbs. The var on rates tied to the arb's duration is the critical risk, then pinning.